Foodstuffs such as scallop adductor muscles and abalones are conventionally dried by being exposed to the sun for about three months. Dried vegetables used in health foods and instant foods are produced by crushing raw vegetables and then drying the vegetable pieces alone or the vegetable pieces mixed with water in a temperature range of 100 to 200° C. Japanese Patent Application Publication No. 7-208862 discloses a vacuum electromagnetic heating dryer. According to the invention disclosed in the publication, an article to be dried (an object) is placed in a pressure-resistant container, the inside of the pressure-resistant container is depressurized, and microwaves are irradiated to the object to heat water molecules therein, thereby drying the object.
However, in the case of sun-drying the foodstuffs, there have been problems of poor yields due to generation of mold and an increase in personnel costs. Moreover, drying the vegetables at high temperatures causes decompositions of vitamins and the like. Furthermore, pursuant to the Japanese Patent Application. Publication No. 7-208862, although depressurization allows the object to be dried at low temperatures, the microwaves are continuously irradiated to the object. Consequently, much heat is applied to the object, causing the object to be deteriorated or even burned in some cases.
Especially when drying food, the use of microwaves allows the drying to be completed in a short time. However, protein and starch are deteriorated by heating even at relatively low temperatures, resulting in changes in constituents of the food. When drying laundry with microwaves, the temperature thereof becomes excessively high, thereby damaging cloth of the laundry or causing the laundry to shrink. When drying lumber using microwaves, inner cellulose is deteriorated, which may hinder the natural properties of the lumber. Moreover, when drying electrical equipment by heating, elements and materials thereof are deteriorated or deformed depending on the heating temperature, which may result in damage of the equipment itself.